The present invention relates to a disconnector which serves as a safety device to be inserted in a drinking water supply pipe to prevent backflow from a downstream point in the piping. In certain systems, the potable water supplied by the pipe may discharge into a contaminated container, and if for some reason that water is forced back upstream, can contaminate water supplied by the source of the drinking water. It is customary to use a check valve to prevent this backflow, but since a small amount of downstream water can pass through a check valve in the upstream direction whenever downstream pressure exceeds upstream pressure, there is a possibility for contamination.
To prevent this occurrence, disconnectors are now used which have two check valves or backflow preventers each having an input port and an output port. An intermediate chamber is connected to the output port of the first check valve and the input port of the second check valve so as to place the check valves in series connection. The input port of the first check valve serves as the input port for the disconnector and the output port of the second check valve serves as the output port of the disconnector. The intermediate chamber has a venting or drain valve controlled by differential pressure between the input port of the first check valve and the intermediate chamber. The venting valve drains the intermediate chamber when the differential pressure between the disconnector's input port and the intermediate chamber falls below a certain value to thereby prevent a backflow from the output side to the input side and potentially contaminate the source. Such a disconnector is disclosed in U.S. Pat. No. 4,478,236.
There is however, a disadvantage in such a disconnector in that at zero flow, normal fluctuations of the pressure at the input port of the disconnector will reduce the pressure differential to a point which causes the venting valve to drain the intermediate chamber. This is inconvenient for two reasons. On the one hand the water in the intermediate chamber is wasted. On the other hand such a water loss delays flow of water through a tap operated thereafter by a user, and may be interpreted as a malfunction by a user unfamiliar with the operation of such a disconnector.